The usual bearing structure of a commercial airplane wing includes an upper skin, a lower skin and two spars connecting both skins. Such spars are commonly made by machining the spar structure (form) out of a metal billet or by fastening the upper and lower chords to a web therebetween.
During design and sizing of spar structures, a plurality of details and analyses must be considered, such as interface with other structural and system components, static strength, stability, fatigue, damage tolerance properties, manufacturing and others.
Airplane wings usually have the spars connected to upper and lower skins by means of fasteners and these four components (front and rear spars) along with the airfoil shape elements (ribs) form the bearing structure of an airplane wing. It is common for such a plurality of structural elements to have conflicting design criteria. For example, the upper skins and upper spar chords must excel in static strength and stability as they are mainly subjected to compression loads. Conversely, the lower skins and lower spar chords are subjected to tension loads and therefore must excel in fatigue and damage tolerance.
Wing spars of commercial transport jets are often machined from an aluminum billet and therefore must meet compression and tension load criteria. Such criteria affect the selection of the spar material and affect the overall structural performance, chiefly increasing weight.